In the manufacture of tube-shaped objects such as plastic tubes a tube which is very soft and hot is discharged from the extruder nozzle of the machine. The tube is cooled externally. Nearest to the nozzle, use is made of a water-cooled cylinder which also constitutes a calibration device for the outside diameter of the tube. A water bath and additional cooling equipment through which the tube is made to pass occurs after the cylinder. Compressed air is introduced into the shaped tube to impart a positive pressure in the tube in order for it to be able to be pressed against the diameter-determining outer cylinder. To maintain the positive pressure in the tube so that it does not collapse a drag plug is provided in the shaped tube at a distance from the nozzle.
The use of compressed air in the tube nevertheless has the disadvantage that the internal surface of the tube readily becomes oxidized. If the air contains water particles these, when impinging upon the interior wall of the tube, may make such marks in the plastic that they in due course can constitute indications of fracture. Moreover, at higher extrusion velocities the plastic tube becomes so hot that the internal surface thereof which is in contact with the compressed air becomes heavily oxidized so that the mechanical strength of the finished tube falls below permissible values. In the manufacture of thick-walled tubes, severe tangential tensile stresses also arise in the material on the inside of the tube, causing the strength to deteriorate. This is due to the fact that the external diameter of the tube is determined with a calibration cylinder at the same time as the tube is cooled severely on the outside while its internal surface is still very hot. As the internal surface gradually cools and shrinks tensile stresses are thus incurred on the inside of the tube, which reduces the ability of the tube to withstand internal positive pressures. Consequently, the tube becomes extra sensitive to the indications of fracture which may arise in the surface of the tube in the form of scratches and other defects. Hitherto, therefore, it has been extremely difficult and occasionally impossible to manufacture certain types of tubes with a specific ratio between wall thickness and external diameter, for instance where this ratio is 15:100.